1. Field of Invention
The present invention relates to a global positioning system (GPS), and more particularly, to a method for obtaining a precise sampling frequency of a GPS.
2. Related Art
A global satellite navigation system is also called a global positioning system (GPS). In the past, the GPS is only limited to military purpose and industrial purpose. With the continuous development of the technology, the GPS begins to be applied to various civil purposes. Generally, the GPS products mainly refer to GPS receivers applied to various purposes, for example, receivers for aviation and voyage purposes, automobile navigation devices, hand-held receivers for mountaineering and entertainment, and other types of communication products. The common GPS product mainly includes an internal antenna, a chipset, external keys, a display panel, and other related parts.
The GPS receiver utilizes satellites orbiting the earth for positioning. Generally, the GPS receiver needs to obtain signals from at least three satellites to calculate the longitude and latitude coordinates of a current position. When the GPS receiver needs to obtain the current position, the GPS receiver compares the time transferred from each satellite. These time differences may inform the GPS receiver of its distance away from each satellite, and therefore the current position is acquired.
The GPS receiver uses an oscillator, such as a temperature compensated crystal oscillator (TCXO), to serve as a reference frequency source required during operation. In other words, the GPS receiver uses the oscillator to generate the reference frequency.
However, as the oscillator is subjected to different manufacturing qualities, as well as the loss of the oscillator after being used for many times, a practically generated sampling frequency is not consistent with the designated sampling frequency given in a specification of the oscillator. In the prior art, when the satellite is tracked, errors of the sampling frequencies are compensated one by one at each data by using a delay locked loop (DLL). However, the GPS receiver consumes much time in the satellite tracking. Moreover, the application of the GPS receiver is limited.